1. Field of the Invention
The present invention relates to a roll forming method for forming a flat tube having a reinforced portion at a center thereof, which is used for a heat exchanger or the like.
2. Description of Related Art
In a radiator for a vehicle, a heat exchanger of a hot water type heating apparatus, or the like, there is used a flat tube through which a fluid for heat exchange flows. The flat tube is brazed to a container (such as a tank) of the heat exchanger.
As a flat tube employed for a heat exchanger having a large width, as shown in FIG. 33, there has been proposed a flat tube 90 having a .theta.-shaped cross section, in which a reinforced portion 91 is provided at a center thereof (as disclosed in JP-A-6-123571). The reinforced portion 91 is composed of edge portions 911 bent perpendicularly at each end of a plate material and a projecting portion 912 folded into a mountain shape at a center portion of the plate material, and both of the edge portions and the projecting portion are connected to each other by brazing.
Further, as a method for continuously cold-forming a long member such as a flat tube on mass production, there has been known a roll forming method.
For example, as disclosed in JP-B2-7-41331, as a method for bending both ends of a belt plate perpendicularly by a roll forming, there has been proposed a forming method composed of separate three processes including a process for bending both ends by approximately 45.degree. a process for bending both ends by approximately 70.degree., and a process for bending both ends by approximately 90.degree..
As described above, because the reinforced portion 91 of the flat tube 90 is, as shown in FIG. 33, composed of the edge portions 911 bent perpendicularly and the projecting portion 912 folded in a mountain shape, those portions should be formed not to create a large gap (depression 925) therebetween, and should be fixedly connected to each other.
If the depression 925 between the edge portion 911 and the projecting portion 912 is large, when the flat tube 90 is brazed to a container 99 such as a tank, the gap which cannot be filled up by brazing still remains, and there occurs a problem of leakage or the like. Therefore, it is necessary to reduce both of an outer curvature radius 921 of the edge portion 911 and a curvature radius 922 at a top portion of the projecting portion 912.
However, it is extremely difficult to perform a roll forming with high accuracy and with high speed while reducing the curvature radii of the above-described edge portion 911 and the projecting portion 912.
For example, when the above-described projecting portion 912 is roll-formed, as shown in FIG. 30, a transverse width of a belt-shaped plate 94 is drawn in (narrowed), and then, as shown in FIG. 31, a projecting portion 95 is formed in a triangular mountain shape. An angle a of the mountain of the projecting portion 95 is gradually made acuter every step of the roll-forming.
However, while being formed, a tensile stress is applied to the top portion of the mountain of the projecting portion 95, and if being formed larger, the top portion is constricted to have a thin thickness. Further, if the curvature of the top portion is made larger, there may occur a problem of fracture or the like. That is, as shown in FIG. 30, the belt-shaped plate is held between rollers and is drawn in from a width direction to form a mountain-shaped projecting portion, a tensile stress T is applied to the top portion of the mountain of the projecting portion 95, as described below.
A first factor constituting the above-described tensile stress T is caused by a deformation resistance when the material is drawn in, and the deforming resistance is due to a shearing resistance of a flat portion (non-mountain portion). That is, a tensile force R (FIG. 31) applied to the top portion per length in a longitudinal direction is expressed as the following formula. EQU R=Sa.times..sigma.a (1)
wherein Sa is a transverse cross section of the flat portion (non-mountain portion), i.e., the transverse width w of the non-mountain portion.times.the plate thickness t, and .sigma.a is a deformation resistance (stress) of the material.
A second factor constituting the above-described tensile stress T is caused by a bending stress .sigma.v of the top portion of the mountain (similarly, per length in the longitudinal direction as shown in FIG. 31), and the tensile stress T is expressed as the following formula with the above-described R. EQU T=(R/t)+.sigma.v=(Sa.times..sigma.a)/t+.sigma.v (2)
When the above-described stress T is larger than the tensile strength of the material, as shown in FIG. 32, a constriction 951 is caused on the top portion (the plate thickness is changed from t to t' by the constriction).
Therefore, it is necessary to roll-form the projecting portion while restricting a drawing amount of the material and a bending amount (curvature). Accordingly, the curvature of the top portion of the mountain cannot be increased excessively, and it is necessary to form by multi-stages, with the result that the number of the processes of the roll forming is increased.